1. Field of Invention
The present invention relates to a liquid crystal apparatus, a driving method thereof, and a projection-type display apparatus and electronic equipment using the liquid crystal apparatus.
2. Description of Related Art
For example, with an active-matrix liquid crystal apparatus, action of writing data to the liquid crystal layer of each pixel is executed by line-at-a-time driving, via switching elements such as a plurality of TFTs (thin-film transistors) connected to a scanning signal line.
Also, in order to eradicate unevenness in the display owing to imbalance in voltage applied to the liquid crystal, and in order to prevent deterioration and so forth of the liquid crystal due to the direct current applied to the liquid crystal, polarity inversion driving is performed, wherein the polarity of the voltage applied to the liquid crystal is inverted at a certain timing.
Polarity inversion driving is a method of driving wherein voltage is applied to one end of the liquid crystal, the polarity (positive or negative polarity) of this voltage is opposite to a reference potential applied to the other end of the liquid crystal.
Incidentally, in the present Specification, the term xe2x80x9cpolarityxe2x80x9d refers to the polarity of the voltage applied to both ends of the liquid crystal. In order to perform polarity inversion driving with an active-matrix type device using TFTs, either the potential applied to the common electrode opposing the pixel electrode across from the liquid crystal is changed, or the potential level of the image data signal is changed with reference to the center potential of the voltage amplitude of the image data signal applied to the pixel electrode.
Known types of polarity inversion driving methods involve inversion by the line wherein polarity inversion is performed each time a scanning signal line is selected, or inversion by the line combined with inversion by the dot wherein polarity inversion is performed for each pixel connected to one scanning signal line.
FIG. 9 and FIG. 10 are models for describing the polarity inversion driving method. With conventional active-matrix liquid crystal apparatus, a polarity inversion driving method has been employed wherein line-at-a-time driving is performed and inversion is performed for each pixel (including for each line), and wherein pre-charging of the data signal lines is performed collectively during the blanking period immediately before.
In FIG. 9 and FIG. 10, S1 through S4 represent data signal lines, and H1 through H4 represent scanning signal lines. The xe2x80x9c+xe2x80x9d and xe2x80x9cxe2x88x92xe2x80x9d for each pixel represent the voltage applied to the liquid crystal of each pixel, and the polarity of the pre-charge potential supplied to the data signal lines immediately prior to the application of the voltage. FIG. 9 represents the voltage polarity of each pixel at field N, and FIG. 10 represents the voltage polarity of each pixel at field N+1. Regarding polarity inversion driving per pixel and per line, the arrangement is such that differing polarity voltage is applied to each neighboring pixel connected to the same data signal line (each neighboring pixel in the vertical direction in FIG. 9 and FIG. 10).
In this case, even when writing the same black data, for example, on the display to two neighboring pixels which are connected to the same data signal line and connected to different scanning signal lines, the signal level for each of the pieces of black data differs, due to the polarity inversion driving. At this time, since the data signal line itself has parasitic capacity, so time is required for changing the potential of the data signal line from the black level potential on the positive polarity side to the black level potential on the negative polarity side.
With reference to FIG. 11 and FIG. 12, description will be made regarding change in the potential of the data signal line when writing the same black data to two neighboring pixels which are connected to the same data signal line.
In FIG. 11, C10 represents the parasitic capacity of the data signal line S1 (i.e., the equivalent capacity of the data signal line S1). Also, the xe2x80x9cxe2x88x92xe2x80x9d and xe2x80x9c+xe2x80x9d noted on the left side of FIG. 11 represents the polarity of the voltage written to the pixels 22 and 24. Incidentally, the pixels 22 and 24 are both to display xe2x80x9cblackxe2x80x9d. The pixels are comprised of a storage capacity and a pixel electrode to which data signals are supplied via a switching element, and a liquid crystal layer to which voltage is applied between the pixel electrode and common electrode.
As shown in FIG. 12, during the horizontal scanning time T1, black level potential B1 is applied to one end of the pixel 22 and black is displayed, and during the next horizontal scanning time T2, black level potential B2 is applied to one end of the pixel 24 and black is displayed, in the same manner. In this case, since a common electrode potential which is set between the black levels B1 and B2 is applied to the other end of the pixels 22 and 24, so that voltage of a negative polarity is applied to the pixel 22, and voltage of a positive polarity is applied to the pixel 24, thus inverting the polarity of the voltage applied to the liquid crystal for the same black display. Moreover, with a normally-white display such as described above, the difference in potential between the black level potentials B1 and B2 is the greatest, as compared with display of other gray scales. Accordingly, in the event that pre-charging is not performed, the parasitic capacity C10 of the data signal line S1 must be charged (or discharged) by the image data signal itself, so as to change the potential of the data signal line from the black level potential B1 to B2, as represented by xe2x80x9cR1xe2x80x9d in the Figure.
Conversely, by performing pre-charging of the same polarity as the polarity of the data signal before supplying the data signal, i.e., by performing pre-charging before the horizontal scanning time T2 so as to maintain the data signal line S1 at the high-voltage second pre-charging potential PV2, as shown as xe2x80x9cR2xe2x80x9d in the Figure, all that is necessary is to change the potential of the data signal line from the second pre-charging potential PV2 to the black level potential B2, so the amount of charging (discharging) of the parasitic capacity C10 of the data signal line S1 does not have to be great. Accordingly, driving of the liquid crystal is increased in speed.
Now, regarding a conventional liquid crystal apparatus, the arrangement has been such that the black level potentials B1 and B2 are respectively set at 1V and 11V, the white level potentials W1 and W2 are respectively set at 5V and 7V, and the pre-charging potentials PV1 and PV2 are respectively set at 4V and 8V. That is to say, the pre-charge potentials PV1 and PV2 have been set symmetrically to the center potential (6V) between the black level potentials B1 and B2, which are the video amplitude.
The 4V and 8V are voltages which are applied to one end of the liquid crystal via a switching element at the time of displaying intermediate gray scale, and are equivalent to the potential level at the time that the T-V curve, which represents the relation between the voltage applied to the liquid crystal (V) and the transmittance of the liquid crystal apparatus (T), becomes the steepest. In other words, 4V and 8V are equivalent to potential levels at the time that the change in transmittance corresponding to change in voltage applied to the liquid crystal is the greatest. By setting the pre-charging potentials PV1 and PV2 as such, the data signal line can be charged or discharged in a short time from the pre-charging potential to a potential for intermediate gray scale display, so accurate intermediate display can be realized even in the event that the sampling period is reduced.
Now, as described above, image display devices have come to be used for various purposes, such as liquid crystal monitors, notebook-type personal computers, and household equipment. Accordingly, development has been proceeded from the perspective of improving precision and portability thereof. For example, regarding improving precision, development has been proceeded toward a display devices with more pixels, e.g., from VGA (640xc3x97480 pixels) to XGA (1024xc3x97768 pixels), from XGA to SXGA (1280xc3x971024 pixels), from SXGA to UXGA (1600xc3x971200 pixels).
The operating frequencies of the above image display devices differ according to the types of image data signals. For example, VGA is used for monitors for notebook personal computers, and the operating frequencies are 60 Hz, 72 Hz, and 75 Hz. SVGA, for example, is used for monitors for notebook personal computers larger than VGA, and the operating frequencies are 56 Hz, 60 Hz, 72 Hz, and 75 Hz. Further, for example, XGA is used for monitors for desktop personal computers and notebook personal computers, and the operating frequencies are 60 Hz, 70 Hz, and 75 Hz. Also, for example, the operating frequency of EWS (SXGA) is 75 Hz.
For example, in the case that a VGA specifications (60 Hz) device is used for a liquid crystal apparatus, there are 800 dot clock signals in 31778 xcexcsec in one horizontal scanning period, having 640 clocks worth within the effective display period. Accordingly, in the event that the aforementioned driving frequencies of 56, 60, 72, and 75 Hz are applied to this device, the period for each horizontal scanning period is shortened. Also, the image data signals input externally can be compressed or extended by digital processing, thereby performing image display corresponding with each of the image data signals.
Also, such liquid crystal apparatus are applied to projectors and the like, and in this case, the arrangement is such that image display can be carried out by performing compression and expansion of the image data signals appropriately, even in the case that the type of image data signal is switched from one to another.
In according with such increase in the number of pixels in image display devices, the size of liquid crystal panels is increasing, and along with this, irregularities in the image on the image display devices is becoming more recognizable. Against the image irregularities, a measure that is improving the uniformity of the pixels and back-lighting has been taken, thereby reducing irregularities in brightness and color.
However, though various steps are being taken regarding the increased frequency which accompanies the increase in the number of pixels, the switching elements in liquid crystal apparatus are comprised of TFTs. Accordingly, there is the problem that the switching properties are slow not only in data signal sampling but also in pre-charging, and accordingly, study is being made regarding various circuit operations accompanying them.
Also, to the scanning signal line, TFT gates serving as switching elements, the number of which is the equal to the number of pixels in the X-direction, are each connected so the capacity component for the scanning""signal line increases. Also, increased panel size means that the wiring resistance of the scanning signals lines increases. Accordingly, the parasitic resistance and parasitic capacity in the scanning signal lines increases and becomes a load, which in turn causes problems of wiring delays.
The present invention has been made in light of the above problems, and it is an object of the present invention to provide a liquid crystal apparatus a driving method thereof, a projection-type display apparatus and electronic equipment using the liquid crystal apparatus, which are capable of preventing deterioration of image quality due to delay of the signal transporting speed when switching, owing to parasitic capacity and parasitic resistance in the supply path of pre-charging signals and parasitic capacity and parasitic resistance in the switching elements.
It is another object of the present invention to provide a liquid crystal apparatus, a driving method thereof, a projection-type display apparatus and electronic equipment using the liquid crystal apparatus, which are capable of preventing deterioration of image quality due to delay of the signal transporting speed when switching, owing to parasitic capacity and parasitic resistance in the scanning signal lines and parasitic capacity and parasitic resistance in the switching elements.
It is a further object of the present invention to provide a liquid crystal apparatus, a driving method thereof, a projection-type display apparatus and electronic equipment using the liquid crystal apparatus, wherein there is no image deterioration even if image data signals of a different type are supplied to the liquid crystal apparatus, by setting the timing for pre-charging and sampling, using the start-up time of the data signal line driving circuit (X-driver) as a reference.
According to one aspect of the present invention, a liquid crystal apparatus is comprised of switching elements, which are electrically connected to pixels , being provided to each of a plurality of pixels formed by crossing a plurality of data signal lines and a plurality of scanning signal lines, driven by inverting the polarity of the voltage applied to the pixels at a predetermined interval, and also comprises: scanning-side driving circuit for sequentially supplying to the plurality of scanning signal lines horizontal scanning signals which turn a plurality of switching elements connected to at least one of the plurality of scanning signal lines turn on during the horizontal scanning period; a plurality of sampling switching circuits which are connected to each of the plurality of data signal lines, sequentially sample data signals during the sampling period, and supply the data signals to each of the plurality of data signal lines; data-side driving circuit for supplying the signals which set the sampling period to the plurality of sampling switching circuits; and a plurality of pre-charging switching circuits which simultaneously pre-charge each of the plurality of data signal lines with a pre-charge potential which has the same polarity as the voltage applied to the liquid crystal layer of the pixels based on the data signals, during the pre-charging period preceding the sampling period wherein the data signals are sequentially supplied to each of the plurality of data signal lines; wherein the time interval from the point at which the pre-charging period ends within a horizontal scanning period to the point that the sampling period of the leading sampling switching circuit is started, is set to be longer than the signal transporting delay time of the sampling switching circuits connected to the data signal line.
According to the one of the embodiments of the present invention, image deterioration can be prevented even in the event that a signal transport delay time occurs in each of the plurality of pre-charging switching circuits following the end of the pre-designed pre-charging period. This is because following all of the pre-charging switching circuits turning off, data sampling to each of a plurality of data signal lines is initiated. Accordingly, even at the data signal lines of which the sampling period is started firstly in the horizontal scanning period in particular, it is possible to avoid the situation in which both of the pre-charging switching circuit and the sampling switching circuits connected thereto turn ON at the same time. Thus, the data signal potential written to the data signal lines is not undesirably affected by the pre-charging potential, and there is no shift in the gray scale value at pixels connected to the data signal lines.
It is preferable that the time interval from the point at which the pre-charging period ends to the point that the leading sampling period within a horizontal scanning period is started is set to be longer than the sum of the time-constants based on the load of each of the pre-charging switching circuits. Thus, the time interval becomes longer than the signal transporting delay time at the pre-charging switching circuit.
The data-side driving circuit can be arranged so as to output the sampling signals after the shift data signal for starting the data-side driving circuit is activated. In this case, the time interval longer than the signal transporting delay time should be set to be the time from the point of end of the pre-charging period to the point to activating the shift data signal.
The present invention may include an adjusting circuit for adjusting and setting the time interval from the point at which the pre-charging period ends to the point that the sampling period of the leading sampling switching circuit within a horizontal scanning period is started.
This adjusting circuit comprises: a counter for counting reference clock signals and being reset by horizontal synchronizing signals; a decoder for decoding the output of the counter and outputting signals for setting the time interval; and a signal generating circuit for generating the pre-charging signals and the shift data signals, based on the output of the decoder. The pre-charging signal and shift data signal can be generated after elapsing of the above time interval, by this adjusting circuit.
Also, the adjusting circuit can fix the time interval, regardless of the driving frequency. Accordingly, image quality never drops, even in the event that image data signals of various types with differing driving frequencies are supplied.
According to another aspect of the present invention, a liquid crystal apparatus is comprised of switching elements, which are electrically connected to pixels, being provided to each of a plurality of pixels formed by crossing a plurality of data signal lines and a plurality of scanning signal lines, is driven by inverting the polarity of the voltage applied to the pixels at a predetermined interval, and also comprises: at least one scanning-side driving circuit for sequentially supplying to the plurality of scanning signal lines horizontal scanning signals which turn ON a plurality of switching elements connected to at least one of the plurality of scanning signal lines during the horizontal scanning period; a plurality of sampling switching circuits which are connected to each of the plurality of data signal lines, sequentially sample data signals during the sampling period, and supply the data signals to each of the plurality of data signal lines; data-side driving circuit for supplying the signals which set the sampling period to the plurality of sampling switching circuits; and a plurality of pre-charging switching circuits which pre-charge each of the plurality of data signal lines with a pre-charge potential which simultaneously has the same polarity as the voltage applied to the liquid crystal layer of the pixels based on the data signals, during the pre-charging period preceding said sampling period wherein the data signals are sequentially supplied to each of the plurality of data signal lines; wherein the time interval from the point at which the (mxe2x88x921)th horizontal scanning period ends to the point that the pre-charging period set within the m""th horizontal scanning period is started, is made to be longer than the signal transporting delay time of the horizontal scanning signal reaching the pixel at the farthest position from the at least one scanning-side driving circuit.
This other aspect of the present invention prevents deterioration of the image quality at the pixel, by taking note of the fact that the signal transporting delay time of the horizontal scanning signal reaching the pixel that is farthest from the scanning-side driving circuit is the longest. With the liquid crystal apparatus, even if the pre-designed (mxe2x88x921)th horizontal scanning period ends, the actual horizontal scanning period of the (mxe2x88x921)th is extended based on the signal transporting delay time. According to the present invention, the pre-charging period of the m""th horizontal scanning period starts after the elapsing of the longest delay time of the signal transportation. Accordingly, the pixels connected to the plurality of switching elements, which is turned ON during the (mxe2x88x921)th horizontal scanning period, are not undesirably affected by the pre-charging potential for the m""th horizontal scanning period.
It is preferable that the time interval, which is from the point of the end of the (mxe2x88x921)th horizontal scanning period until the point of the start of the pre-charging period being set within the m""th horizontal scanning period, is set to be longer than the sum of the time-constants based on each of the loads of one scanning signal line and the switching element of the farthest pixel. Thus, the above mentioned time interval can be made to be longer than the signal transporting delay time of the horizontal scanning signal reaching the pixel that is farthest from the scanning-side driving circuit.
The present invention can include an adjusting circuit for adjusting and setting the time interval from the point at which the (mxe2x88x921)th horizontal scanning period ends to the point that the pre-charging period being set within the m""th horizontal scanning period is started.
The adjusting circuit comprises: a counter, which counts reference clock signals and is reset by horizontal synchronizing signals; a decoder for decoding the output of the counter and outputting signals for setting the time interval; and a signal generating circuit for generating the pre-charging signals and the shift data signals, based on the output of the decoder. According to this adjusting circuit, the pre-charging signal for the m""th horizontal scanning period can be generated, after the above time interval elapses following the end of the (mxe2x88x921)th horizontal scanning period.
Also, the adjusting circuit can fix the time interval, regardless of the driving frequency. Accordingly, image quality never drops, even in the event that image data signals of various types with differing driving frequencies are supplied.
Each of the above-described inventions can comprise a pair of substrates with the liquid crystal sandwiched therebetween, wherein the plurality of sampling switching circuits can be formed by a plurality of switching elements provided on one of the pair of substrates. Such switching element may be a MOS transistor or thin-film transistor.
Also, applying the present invention to a projection-type display device or electronic equipment having a liquid crystal apparatus with the above characteristics can prevent image deterioration therein.